This invention relates to a method for precharging the negative electrodes of nickel-cadmium alkaline storage cells, and is particularly useful in sealed, semi-sealed or semi-open cells.
Precharging the negative electrode of a cell consists in establishing a state of charge such that the charged capacity of the negative electrode is greater than the charged capacity of the positive electrode. To avoid the evolution of hydrogen in sealed cells it is known to provide the negative electrode with a capacity greater than that of the positive electrode, so that when the positive electrode reaches its completely charged state there still remains some non-charged capacity on the negative electrode. If the negative electrode is charged by an amount which is less than the difference between the negative and positive capacities before the positive electrode is charged, some uncharged capacity will remain on the negative electrode at the end of charging the positive electrode. Furthermore, after the positive electrode is discharged, the negative electrode will still be partly charged. As a result, the end of discharge is clearer, and the capacity of cells with precharged negative electrodes is more stable during cycling.
It would appear fairly simple to impart these states of charge by charging the cell while in the open state with the required capacity, but in cells with thin electrodes which are close together, it is known that when the positive electrode reaches the end of charging, the charge efficiency of the negative electrode becomes very low or even zero. The quantity effectively charged on the negative electrode therefore does not correspond to the quantity of electricity supplied.
On the other hand, if it is sought to impart the required charge to the negative electrode before assembling the cell, it is found that the charge does not remain stable during handling of the electrode, due to the reactivity of reduced cadmium.
It is known through U.S. Pat. No. 3,713,889 to fix the relative states of charge of the electrodes of an alkaline storage cell in which the negative electrode has a capacity greater than that of the positive electrode by charging the cell in the open state and by imparting a sufficient overcharge to it, after complete charging of the positive electrode, for the negative electrode to be in a state of charge such that its charged capacity is greater than that of the positive electrode. Since this excess charged capacity must be fixed accurately, the previously-mentioned overcharge is carried out at a temperature of about -10.degree. C. Preferably, the excess charged capacity of the negative electrode lies between 10 and 50% of the difference between the real overall capacity of the negative electrode and the capacity of the positive electrode. Such a method is difficult, however, since it is necessary to operate at low temperature.
Further, in some cases where the required precharge is to be made on the positive electrode, it has been proposed to introduce a chemical oxidizing agent which transforms nickel hydroxide into higher hydroxides. However, transposition of this method to precharging the negative electrode by means of cadmium hydroxide reducers is not possible, since cadmium hydroxide is chemically much too stable.